1. Field of the Invention
The present invention relates to photosensitive resin compositions which may be used for photoresist, and more specifically relates to photosensitive resin compositions which may be used for photoresist of laser direct image formation type photoresist to produce an electroconductive circuit or an electrode pattern substrate formed by a metal such as copper etc. or an electroconductive metal oxide, such as ITO etc.
2. Description of the Prior Art
Photosensitive compositions are widely used for various uses, for example, for photoresist, plate materials for lithography or letterpress, presensitized plates (PS plates) for offset printing, information recording materials, materials for making relief image, and the like.
There are problems that although many of these photosensitive compositions are photosensitive to ultraviolet rays, the photographic sensitivity thereof is generally several tens to several hundreds mJ/cm.sup.2 and thus light sources of high output are required and moreover, the efficiency of energy conversion to the imaging is poor. On the other hand, there is an image-forming method wherein an image is directly formed using a light of high energy density such as laser. This method has not only an advantage that energy conversion efficiency becomes better but an advantage that image-forming steps can greatly be simplified. As for light sources for scanning exposure in direct image formation, it is advantageous to use visible light laser, which gives emission of radiation stable in lifetime and intensity, rather than ultraviolet laser. Thus, there has been desired the emergence of visible light sensitive composition having a sensitivity such that scanning exposure by visible light laser is possible, and many visible light sensitive compositions having a high sensitivity to Ar.sup.+ laser, which has a stable emission of radiation in a visible region of wavelength 488 nm, have been proposed (please refer, for example, to Japanese Laid-Open Patent Publications Nos. 31848/1987, 233736/1986 and 221403/1985, European Patent 255486, Printed Circuit World Convention IV (1987, June), technical paper 22-2-22-13 Gists of the 1st Academic Lecture Meeting of the Printed Circuit Society, pages 91-92 etc.).
These visible light sensitive compositions are used in dry film form and liquid form. However, they have disadvantages mentioned as follows.
Dry film resist consists usually of a light transmissive support film, a sticky photosensitive resin layer formed thereon, and its protecting film. A pattern forming process using a dry film resist consists of firstly peeling off the protecting film from the dry film, sticking the photosensitive resin layer onto a substrate by pressing, irradiating visible light laser to record an image pattern based on CAD data through the light transmissive support film, then conducting a developing treatment with a developer, which dissolves selectively non-exposed portions to form a desired pattern on the substrate.
Moreover, in case the image-formed substrate is an etchable material such as copper laminated plate etc., an etched pattern may be formed by treating the substrate by etchant. Nowadays, however, electronic circuits became more highly dense, requiring higher resolution of less than 150 .mu.m line width for circuit pattern formation, and a dry film resist cannot meet the requirements any more, because a dry film resist is exposed to a light through a light transmissive support film, which should be as thin as possible to obtain a high resolution but need to be somewhat thick, namely 15-50 .mu.m in general, to function as a support on which a photosensitive resin layer is coated, and consequently a high resolution is difficult to obtain.
On the other hand, the problem of resolution may be solved, if a liquid form photoresist is used. However, it was difficult to obtain a highly sensitive resist by the conventional technology. In case light exposure is conducted by laser direct image formation process (LDI process), the sensitivity decreases markedly by receiving curing inhibition by oxygen in air, because it is conducted in the ambient atmosphere. In order to solve the problem, countermeasures such as coating said resist by a cover coat comprising mainly of water soluble or alkaline water soluble polymer having high oxygen barrier property such as polyvinyl alcohol etc. have been taken. However, it is the actual circumstances that there is no photosensitive resin composition having sufficient sensitivity.
From the capacity of the current laser direct image forming apparatus, the amount of light exposure to form a resist film, which can stand against etching, is necessary to be less than 5 mJ/cm.sup.2, in order to be used industrially.
The present inventors made an intensive study about a resist for pattern formation without problems as mentioned above. As a result, they have found highly photosensitive compositions which can form a highly sensitive resist, on which a direct image formation with laser is possible, surprisingly even without coating an oxygen barrier cover coat and completed the present invention.